To recap things: The company Harmonix developed Guitar Hero 1 and 2. Activision published them. MTV bought Harmonix away from Activision. Now Neversoft and Vicarious Visions develop Guitar Hero. Through EA, Harmonix made Rock Band, an expansion on the GH formula that added drums, bass and vocal playstyles.
Guitar Hero 4, or rather Guitar Hero: World Tour was recently announced, with the press release stating, "For the first time, guitar players can also play bass, play drums and sing vocals!"
It also seems that the controllers for each game may be incompatible with the other. In other word, you'd have to pay almost $400 if you wanted to play two songlist's worth of what is essentially the same game.
I have no plans to get World Tour but do plan to get Rock Band and its announced sequel. The only positive announcement has been Activision announcing the "Create-A-Song" feature for World Tour, giving players a music-making application and letting them upload their creations online for all to play. If Activision wants Guitar Hero to stay a viable property, they need to do more like this, and add unique and creative features that Rock Band didn't beat to the punch.